The present invention relates to induction heating and melting systems that use magnetic induction to heat a crucible in which metal can be heated and/or, melted and held in the molten state by heat transfer from the crucible.
Induction melting systems gain popularity as the most environmentally clean and reasonably efficient method of melting metal. In the induction melting furnace 1 shown in FIG. 1, the electromagnetic field produced by AC current in coil 2 surrounding a crucible 3 couples with conductive materials 4 inside the crucible and induces eddy currents 5, which in turn heat the metal. As indicated in FIG. 1, the arrows associated with coil 2 generally represent the direction of current flow in the coil, whereas the arrows associated with eddy currents 5 generally indicate the opposing direction of induced current flow in the conductive materials. Variable high frequency ac (typically in the range from 100 to 10,000 Hz) current is generated in a power supply or in a power converter 6 and supplied to coil 2. The converter 6, typically but not necessarily, consists of an AC-to-DC rectifier 7, a DC-to-AC inverter 8, and a set of capacitors 9, which, together with the induction coil, form a resonance loop. Other forms of power supplies, including motors-generators, pulse-width modulated (PWM) inverters, and the like, can be used.
As shown in FIG. 2, the magnetic field causes load current 10 to flow on the outside cylindrical surface of the conductive material, and coil current 11 to flow on the inner surface of the coil conductor. Crucible 3 in a typical furnace is made from ceramic material and usually is not electrically conductive. The efficiency of the furnace is computed by the formula:                     η        =                  1                      1            +                                                            D                  1                                                  D                  2                                            ·                                                ρ                  1                                                  ρ                  2                                            ·                                                Δ                  2                                                  Δ                  1                                                                                        equation        ⁢                  xe2x80x83                ⁢                  (          1          )                    
where
xcex7=furnace efficiency;
D1=coil inner diameter;
D2=load outer diameter;
xcfx811=resistivity of coil winding material (copper);
xcfx812=resistivity of load (melt);
xcex941=current depth of penetration in copper winding; and
xcex942=current depth of penetration in load (melt).
The depth of current penetration (xcex94) is a function of a material""s properties as determined by the formula:                     Δ        =                  k          ·                                    ρ                              f                ·                μ                                                                        equation        ⁢                  xe2x80x83                ⁢                  (          2          )                    
where:
xcfx81=resistivity in ohmxc2x7meters;
f=frequency in Hertz;
xcexc=magnetic permeability (dimensionless relative value); and
xcex94=depth of penetration in meters.
The constant, k=503, in equation (2) is dimensionless.
Because current does not penetrate deep into the low resistivity copper material of the coil, the typical coil efficiency is about 80 percent when the molten material is iron. Furnaces melting low resistivity materials such as aluminum (with a typical resistivity value of 2.6xc3x9710xe2x88x928 ohmxc2x7meters), magnesium or copper alloys have a lower efficiency of about 65 percent. Because of significant heating due to electrical losses, the induction coil is water-cooled. That is, the coil is made of copper tubes 12 and a water-based coolant is passed through these tubes. The presence of water represents an additional danger when melting aluminum, magnesium or their alloys. In case of crucible rupture, water may get into molten aluminum and a violent chemical reaction may take place in which the aluminum combines with oxygen in the water, releasing free hydrogen which may cause an explosion. Contact between water and magnesium may similarly result in an explosion and fire. Extreme caution is taken when aluminum or magnesium is melted in conventional water-cooled furnaces.
An object of the present invention is to improve the efficiency of an induction furnace by increasing the resistance of the load by using as the load a crucible made of a high temperature electrically conductive material or a high temperature material with high magnetic permeability. It is another object of the present invention to improve the efficiency of an induction furnace by reducing the resistance of the induction coil by using as the coil a cable wound of multiple copper conductors that are isolated from each other. It is still another object of the invention to properly select operating frequencies to yield optimum efficiency of an induction furnace.
It is a further object of the present invention to provide a high efficiency induction melting system with a furnace and power supply that do not use water-cooling and can be efficiently air-cooled.
In its broad aspects, the present invention is an induction furnace that is used for melting a metal charge. The furnace has a crucible formed substantially from a material having a high electrical resistivity or high magnetic permeability, preferably a silicon carbide or a high permeability steel. At least one induction coil surrounds the crucible. The coil consists of a cable wound of a plurality of conductors isolated one from the other. An isolation sleeve electrically and thermally insulates the crucible from the at least one induction coil. Preferably, the isolation sleeve is a composite ceramic material, such as an air-bubbled ceramic between two layers of ceramic. In alternate examples of the invention, the induction furnace is used to heat the metal charge to a temperature that may be below its melting point.
Copper is especially preferred for the conductors, because of its combination of reasonably high electrical conductivity and reasonably high melting point. A preferred form of the cable is Litz wire or litzendraht, in which the individual isolated conductors are woven together in such a way that each conductor successively takes all possible positions in the cross section of the cable, so as to minimize skin effect and high-frequency resistance, and to distribute the electrical power evenly among the conductors.
In another aspect, the present invention is an induction melting system that is used for melting a metal charge. The system has at least one power supply. The crucible that holds the metal charge is formed substantially from a material having a high electrical resistivity or high magnetic permeability, preferably a silicon carbide or a high permeability steel. At least one induction coil surrounds the crucible. The coil consists of a cable wound of a large number of copper conductors isolated one from the other. An isolation sleeve electrically and thermally insulates the crucible from the at least one induction coil. Preferably, the isolation sleeve is a composite ceramic material, such as an air-bubbled ceramic between two layers of ceramic. Preferably, the induction melting system is air-cooled from a single source of air that sequentially cools components of the power supply and the coil. The metal charge is placed in the crucible. Current is supplied from the at least one power supply to the at least one coil to heat the crucible inductively. Heat is transferred by conduction and/or radiation from the crucible to the metal charge, and melts the charge. In alternate examples of the invention, the induction furnace is used to heat the metal charge to a temperature that may be below its melting point.
These and other aspects of the invention will be apparent from the following description and the appended claims.